Today, virtually all high power mobile systems use an energy source based on the combustion of a fuel with air to generate a gas at a high temperature and pressure. The high pressure, high temperature gas is allowed to expand and do work on pistons or turbine blades that extract the desired energy. These energy sources are categorized as heat engines. The pistons and turbines are metallic structures with dedicated mass and bulk specifically designed for the extraction of energy. These devices cover a broad range of energy production per unit mass. The lightest devices achieve an energy production in the range of 0.50 watts per gram. The maximum efficiency of heat engines seldom exceeds approximately 40%. This efficiency is achieved only within a narrow range of operating conditions.
Electrically powered vehicles are an alternative to heat engines, but have drawbacks of their own. Vehicles which employ photovoltaic cells to generate electricity are well-known. These vehicles require sunlight to generate electricity using the photovoltaic cells. Such vehicles are of limited usefulness in the absence of light. For example, U.S. Pat. No. 5,518,205 discloses a solar powered aircraft. This aircraft employs means for positioning the solar cells for optimal sun exposure. Energy is stored for night use in batteries. Alternatively, energy is obtained at night by using a hydrogen/oxygen fuel cell located inside the aircraft.
U.S. Pat. No. 5,348,254 discloses a long period floating airship which employs a solar cell battery during the daytime, and requires the use of a hydrogen engine during the night.
U.S. Pat. No. 4,907,764 discloses an aircraft with external cells powered by long wavelength infra-red radiation from earth.
U.S. Pat. No. 4,768,738 discloses a solar powered aircraft having a flexible skin capable of transforming solar energy into electricity. The electrical energy must be stored by the use of a battery.
U.S. Pat. No. 4,415,133 discloses a solar powered aircraft having photovoltaic cells on the surface of the wings. Again, batteries must be used to store energy for night use.
U.S. Pat. No. 4,364,532 discloses a collector for high altitude collection of solar energy to be used to power a balloon, or airship.
Another option for energy generation is battery generation. In this regard, U.S. Pat. No. 3,937,424 discloses energy generation by use of a battery which forms a portion of the wing.
Fuel cells offer a viable energy generating alternative to heat engines, solar power and battery storage. Fuel cells, in their simplest form, contain two electrodes layers encasing an electrolyte layer. An oxidizing fuel (liquid or gas) is placed on one side of the cell, and a reducing fuel (liquid or gas) is placed on the other side of the cell. The electrolyte between the electrodes can be for example, solid oxides, molten salts, or solid polymer electrolytes. Solid polymer electrolytes are particularly useful because they do not require the special encapsulation measures needed for molten salt electrolytes. Further, solid polymer electrolytes are operable at low temperatures unlike solid oxide electrolytes.
Simple fuel cells use oxygen and hydrogen as the fuels. On the hydrogen side of the cell, hydrogen reacts to produce electrons and hydrogen ions as follows: ##EQU1## On the oxygen side of the cell, oxygen reacts with electrons and water in the electrolyte to form hydroxyl ions: ##EQU2## Within the electrolyte, hydroxyl ions and hydrogen ions react to form water: ##EQU3## The electrons flow through the external circuit forming a power source.
The fuel cell energy generating process is not limited by the same thermodynamic factors that limit heat engines. Fuel cell efficiency is nearly linear with load. When the power drain is low, the efficiency approaches 100%. Valuable levels of output can be achieved at efficiencies between 40 and 80%.
In the context of energy production for vehicles, these cells are generally stacked to form an electricity generating assembly (see, e.g., U.S. Pat. No. 4,175,165). Typically, fans or ducts are necessary to force air over the cells to produce energy.